Flexibility and plasticity in neuronal signalling is a property of the central nervous system, which allows the individual to adapt to its surrounding and to change its behavior according to past experience. Those electrophysiological phenomena in which the response of a stimulated pathway is modified by its preceding activity are likely to underlie the neuronal plasticity. An example of such a phenomenon is postactivation potentiation, which is characterized by an increase in the neuronal activity to a single stimulus following a brief intense synaptic stimulation. The dentate fascia (of the hippocampal formation) following brief tetanic stimulation of its afferent pathway (the perforant path) yields potentiation lasting for hours, even days. The time course makes the potentiation comparable to the duration of a learned response. In view of the possibility that postactivation potentiation and memory formation may have a common mechanism, we studied with the electron microscope morphological changes on synapses in the dentate fascia in the course of postactivation potentiation. Results of these experiments showed a considerable long-lasting swelling of the dendritic spines in the region of the dentate fascia where the perforant path terminates. The degree, the location and the time course of the poststimulation morphological change points to a causal rather than a coincidental relation between the spine swelling and postactivation potentiation. If the spine swelling were the common mechanism of potentiation and memory, then it should also occur in the process of task acquisition. Therefore, this proposal is ultimately aimed at the demonstration of morphological changes underlying certain phases of the process of learning and memory. BIBLIOGRAPHIC REFERENCES: Fifkova, E. and Van Harreveld, A. (1977) Long-lasting morphological changes in dendritic spines of dentate granular cells following stimulation of the entorhinal area. J. Neurocytology, 6: 211-230. Fifkova, E., Van der Wege, B. J. and Van Harreveld, A. (1977) Ultrastructural changes in the dentate molecular layer during conditioning. VIIth Annual Meeting Soc. for Neurosci. In press.